Conventional AC plasma display technology includes display panels comprising two glass plates having orthogonally positioned conductor arrays thereon encapsulated in a gas envelope, the intersections of said conductor arrays forming gas cells. The conductor arrays are overcoated with a dielectric and insulated from the gas and thus capacitively coupled to the gas in the panel. When signals exceeding the ionization potential of a pair of conductors occur during a write operation, a discharge takes place and a wall charge potential is formed on the cell walls. This potential combines with a lower level sustain signal to continually discharge the cell at a relatively high frequency (40 KHz) to maintain the discharge. Erase takes place by neutralizing the wall charge and thereby removing the wall charge potential.
The operation of an AC plasma display panel thus requires the application of sequences of three control signals, i.e., sustain, write and erase. These signals are applied to drivers which control the energization state of the illuminable cells in the plasma panel display and are sequenced so as to provide the sustain, write and erase operations required in the plasma panel display. The sustain operation has two separate applications. The first application, as described above, is to maintain the information on the plasma panel display in its then present state. The second application is to normalize a write or an erase operation by a sustain sequence. If the sustain sequence is not properly applied before and after write and erase operations, then a write or erase operation will not be successfully completed.
A plasma panel display may be controlled by a data processing system or controller which serves two purposes in relation to the display. First, it sends data signals which are representative of the information that is to be displayed. Second, it sends the control commands, such as write or erase, which cause the information to be displayed by or erased from the plasma panel display. These control commands are received by the plasma panel through appropriate control circuitry and are operated upon so as to effect the appropriate control operations of write, sustain and erase.
One method of accomplishing such plasma panel control is disclosed in U.S. Pat. No. 3,851,211 where individual control sequences of the sustain, write and erase signals are stored within a ROS. Logic circuitry within the panel assembly but external to the ROS receives the control information from a data processing system or controller. The logic circuitry then selectively activates the appropriate control sequences of sustain, write and erase within the ROS so as to effect control of the plasma panel display.
As the cost of storage continues to decline, it would be desirable to provide control of the individual operations of sustain, write and erase within a storage device located in the plasma panel assembly. By so doing, the external logic circuitry required in the prior art is simplified.
As previously described, the application of sequences of the sustain signal has two functions. The first function is to continuously discharge the cells in the plasma display so as to maintain the discharge. The second function is to normalize a write or an erase operation, i.e., to successfully complete a write or an erase operation. The sustain signal operates at a relatively high frequency of 40 kilohertz. On the other hand, the erase and write signals operate at a much lower frequency, around 20 kilohertz.
At this point, a digression is in order to explain the relationship between frequency and repetition rate. As is well known, frequency is defined as the reciprocal of the period of a periodic waveform. However, when one has a signal composed of sequences having a multiplicity of periods, it is no longer correct to speak in terms of frequency for the entire signal. Instead, the proper term describing this type of signal is repetition rate. The repetition rate is found by averaging the individual periods found in a nonperiodic waveform and taking the reciprocal of them. Note that for a periodic waveform, the frequency will be equal to the repetition rate.
The individual sequences of the sustain signal, erase signal, and write signal, are periodic. As previously disclosed, the sustain signal has a frequency of 40 kilohertz. The erase and write signals are somewhat slower having a frequency of 20 kilohertz. In order to carry out an erase or a write operation, the erase or write signals must be followed by sequences of sustain signals. Thus, for example, an erase function comprises sequences of the erase signal combined with sequences of the sustain signal. In this type of situation, the waveform comprising the erase function must be characterized by use of the term repetition rate since it includes signals of different frequencies.
The brightness of the illuminable cells in a plasma panel display are directly proportional to the repetition rate of control signals applied thereto. A problem that arises as a result of this phenomenon and the disparity in frequency between the sustain, and the write and erase signals is that the plasma panel becomes dimmer during long periods of write and erase functions. This occurs because the average repetition rate of control signals applied to the cells decreases over the average repetition rate during a sustain function.
Several attempts have been made to suppress this dimming effect during write or erase sequences, however none of them are totally satisfactory. One approach involved requiring the display panel to be deenergized during a time interval immediately preceding an erase function. However, this approach did not eliminate dimming, but only reduced a portion of it. Another approach involved reducing the repetition rate during a write sequence until the light is substantially unperceivable in a room ambient light background. After the write is completed, the repetition rate is increased until the panel is once again visually perceivable. Both of these approaches are unattractive since they require the plasma panel to deviate from its normal operating mode during writing or erasing.
This problem is solved in the instant invention by mixing high frequency sustain signals with erase and write signals chosen so that the average repetition rate of the signals comprising an erase function and a write function are equal to the average repetition rate of the signals comprising the sustain function. By having a uniform repetition rate for the functions of sustain, erase and write, no dimming occurs.
It would be desirable when operating a gas panel to be able to vary the average brightness of the panel. With this capability, one would be able to adjust the brightness of a plasma panel display so as to improve perceptibility in different ambient light backgrounds. It would also be most desirable to combine the brightness modulation control with the antiflicker mechanism. This is accomplished in the instant invention by storing in a ROS a multiplicity of sustain, write and erase signals. The various signals may be combined to produce an average repetition rate at a given brightness level. This suppresses flicker and provides the capability of mixing average repetition rate signals at a given brightness level to produce multiple brightness levels without flicker.
Accordingly, it is an object of this invention to provide improved ROS control of a plasma panel display.
It is another object of this invention to provide a plasma panel display wherein flicker is substantially eliminated.
It is still another object of this invention to provide various controllable brightness levels in a plasma panel display, all without flicker.